U.S. patent application number 12/304602 was filed with the patent office on 2009-08-06 for vibrator, resonator using the same and electromechanical filter using the same.
This patent application is currently assigned to PANASONIC CORPORATION. Invention is credited to Akinori Hashimura, Michiaki Matsuo, Kunihiko Nakamura, Yoshito Nakanishi.
Application Number | 20090195330 12/304602 |
Document ID | / |
Family ID | 38831804 |
Filed Date | 2009-08-06 |
United States Patent
Application |
20090195330 |
Kind Code |
A1 |
Nakamura; Kunihiko ; et
al. |
August 6, 2009 |
VIBRATOR, RESONATOR USING THE SAME AND ELECTROMECHANICAL FILTER
USING THE SAME
Abstract
An object is to provide a resonator and a vibrator with a high Q
value in which dissipation of vibration energy in vibration of the
vibrator is small, and a thickness of a support part of the
vibrator of a beam structure is made thicker than a thickness of
the vibrator and the support part is formed in axisymmetry with
respect to a length direction of a beam. By this configuration,
brittleness of the support part is improved and loss of vibration
energy from the support part is reduced and also loss of vibration
energy resulting from surface roughness of a surface of the
vibrator can be reduced, so that a resonator having a high Q value
can be provided.
Inventors: |
Nakamura; Kunihiko; (Osaka,
JP) ; Matsuo; Michiaki; (San Jose, CA) ;
Nakanishi; Yoshito; (Osaka, JP) ; Hashimura;
Akinori; (Portland, OR) |
Correspondence
Address: |
PEARNE & GORDON LLP
1801 EAST 9TH STREET, SUITE 1200
CLEVELAND
OH
44114-3108
US
|
Assignee: |
PANASONIC CORPORATION
Osaka
JP
|
Family ID: |
38831804 |
Appl. No.: |
12/304602 |
Filed: |
June 14, 2007 |
PCT Filed: |
June 14, 2007 |
PCT NO: |
PCT/JP2007/062033 |
371 Date: |
February 19, 2009 |
Current U.S.
Class: |
333/200 ;
327/44 |
Current CPC
Class: |
H03H 2009/02314
20130101; H03H 9/1057 20130101; H03H 9/2405 20130101; H03H
2009/02519 20130101; H03H 3/0072 20130101; H03H 9/2463 20130101;
H03H 9/2457 20130101; H03H 9/02244 20130101 |
Class at
Publication: |
333/200 ;
327/44 |
International
Class: |
H03H 9/24 20060101
H03H009/24; H03H 9/125 20060101 H03H009/125 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 14, 2006 |
JP |
2006-164382 |
Claims
1. A vibrator unit, comprising: a cavity formed in a back surface
of a single-crystal substrate; and a vibrator formed so that at
least one end of the vibrator is supported by a support part of the
single-crystal substrate within a space between a front surface of
the single-crystal substrate and a bottom surface of the cavity,
wherein the single-crystal substrate is a silicon substrate;
wherein a cross section shape of the beam type vibrator is a
triangular shape or a trapezoidal shape surrounded by crystal
planes of {100} and {111}; and wherein a thickness of the vibrator
is thinner than that of the support part and the support part is
axisymmetrical with respect to a length direction of a beam of the
vibrator.
2. A resonator comprising: a single-crystal substrate; a cavity
formed in a back surface of the single-crystal substrate; a beam
type vibrator formed so that at least one end of the vibrator is
supported by a support part of the single-crystal substrate within
a space between a front surface of the single-crystal substrate and
a bottom surface of the cavity; and an electrode which applies
excitation force of electrostatic force to the beam type vibrator,
wherein the single-crystal substrate is a silicon substrate;
wherein a cross section shape of the beam type vibrator is a
triangular shape or a trapezoidal shape surrounded by crystal
planes of {100} and {111}; and wherein a thickness of the vibrator
is thinner than that of the support part, and the support part is
axisymmetrical with respect to a length direction of a beam of the
vibrator.
3. The resonator according to claim 2, wherein a plurality of the
beam type vibrators are formed within the space between the front
surface of the single-crystal substrate and the bottom surface of
the cavity.
4. The resonator according to claim 2, wherein the single-crystal
substrate is an SOI layer of an SOI substrate.
5. (canceled)
6. (canceled)
7. (canceled)
8. The resonator according to claim 2, wherein the electrode is
opposed to the whole area of a width direction of a side surface of
the beam type vibrator exposed to a front surface of the
single-crystal substrate through an air gap; and wherein a capacity
is formed between the beam type vibrator and the electrode.
9. The resonator according to claim 8, wherein conductivity of the
vibrator is asymmetrical with respect to a torsional center axis of
the longitudinal direction of the beam of the vibrator.
10. The resonator according to claim 2, wherein the electrode is
opposed to substantially a half area of a width direction of a side
surface of the beam type vibrator exposed to a front surface of the
single-crystal substrate through an air gap; and wherein a capacity
is formed between the beam type vibrator and the electrode.
11. The resonator according to claim 2, wherein the plurality of
electrodes are arranged in response to a resonance mode order of
the beam type vibrator.
12. The resonator according to claim 2, wherein the electrode is
fixed on a thick film part of the single-crystal substrate arranged
at a peripheral edge of the cavity through an insulating film.
13. The resonator according to claim 2, wherein the beam type
vibrator is a straddle mounted beam and both ends of its beam are
equipped with support parts made of impurity diffusion regions of a
conduction type opposite to that of the beam type vibrator, and
wherein the beam type vibrator is used as a channel of an amplifier
and the support parts made of the impurity diffusion regions are
used as a source region and a drain region of the amplifier.
14. The resonator according to claim 2, comprising a plurality of
resonators electrically arranged in parallel.
15. The resonator according to claim 2, wherein the vibrator is
accommodated inside a case in which the atmosphere is sealed in a
vacuum.
16. An electromechanical filter constructed so as to pass or block
a signal of a desired frequency band by using the resonator
according to claim 2.
Description
TECHNICAL FIELD
[0001] The present invention relates to a vibrator, a resonator
using the same and an electromechanical filter using the same, and
particularly to implementation of a high-performance filter circuit
inside an electrical circuit integrated at a high density.
BACKGROUND ART
[0002] A mechanical resonator using a microfabrication technique
has received attention in recent years.
[0003] One example of a conventional mechanical resonator will be
described with reference to FIG. 33. FIG. 33 is a simplified
diagram showing a configuration of a mechanical vibration filter
using flexural vibration proposed in Non-patent Reference 1.
[0004] This filter is formed by patterning on a silicon substrate
using a thin film process, and is constructed of an input rail 104,
an output rail 105, straddle mounted beams 101, 102 placed at an
air gap of 1 .mu.m or less with respect to the respective rails,
and a coupling beam 103 for coupling the two beams. A signal
inputted from the input rail 104 is capacitively coupled to the
beam 101 and generates electrostatic force in the beam 101. A
filtering output of the input signal is constructed so as to be
fetched by exciting mechanical vibration only when a frequency of
the signal matches with the vicinity of a resonance frequency of an
elastic structural body made of the beams 101, 102 and the coupling
beam 103 and further detecting this mechanical vibration as a
change in capacitance between the output rail 105 and the beam
102.
[0005] In the case of being set at an elastic modulus E, a density
.rho., a thickness h and a length L for a straddle mounted beam
with a rectangular cross section, a resonance frequency f of
flexural vibration is shown by the following formula.
f = 1.03 h L 2 E .rho. [ Mathematical formula 1 ] ##EQU00001##
[0006] When a material is set at polysilicon, it becomes E=160 GPa
and .rho.=2.2.times.10.sup.3 kg/m.sup.3 and also when dimensions
are set at L=40 .mu.m and h=1.5 .mu.m, it becomes f=8.2 MHz and a
filter with a band of about 8 MHz can be constructed. Steep
frequency selection characteristics with a high Q value can be
obtained by using mechanical resonance as compared with a filter
constructed of a passive circuit such as a capacitor or a coil.
[0007] However, in the construction described above, there are the
following restrictions in the case of constructing a filter having
a higher frequency band. That is, it is apparent from (mathematical
formula 1) that it is first desirable to change a material and
increase E/.rho., and when E is increased, the amount of
displacement becomes small even when force of flexing a beam is
equal, and it becomes difficult to detect displacement of the
beam.
[0008] Also, when an index representing flexibility in a beam is
set at a ratio d/L between a length L of the beam and the amount d
of flexure of the beam center at the time of applying a static load
to a beam surface of a straddle mounted beam, d/L is expressed by a
proportional relation of the following formula.
d L .varies. L 3 h 3 1 E [ Mathematical formula 2 ]
##EQU00002##
[0009] From these, in order to increase a resonance frequency while
holding a value of d/L, at least E cannot be changed and it is
necessary to obtain a material with a low density .rho., and it is
necessary to use a composite material such as CFRP (Carbon Fiber
Reinforced Plastics) as the material with a low density at a
Young's modulus equal to that of polysilicon. In this case, it
becomes difficult to construct a micromechanical vibration filter
in a semiconductor process.
[0010] Hence, a second method without using such a composite
material includes a method for increasing hL.sup.-2 by changing a
dimension of a beam in (mathematical formula 1). However, an
increase in the thickness h of the beam and a decrease in the
length L of the beam result in a decrease in d/L of (mathematical
formula 2) which is an index of flexibility and it becomes
difficult to detect flexure of the beam.
[0011] When a relation between log(L) and log(h) is shown in FIG.
34 with respect to (mathematical formula 1) and (mathematical
formula 2), a straight line a has a relation obtained from
(mathematical formula 1) and a straight line b has a relation
obtained from (mathematical formula 2). In this FIG. 34, in the
case of selecting h and L in the range (region A) upper than a
straight line with a gradient "2" using a point A of the present
dimension as an initial point, f becomes large and in the case of
selecting h and L in the range (region B) lower than a straight
line with a gradient "1", d/L becomes large. Therefore, the hatch
portion (region C) in FIG. 34 is in the range of h and L capable of
increasing a resonance frequency while ensuring the amount of
flexure of the beam.
[0012] It is apparent from FIG. 34 that miniaturization of both
dimensions of the length L of the beam and the thickness h of the
beam is a necessary condition in a higher frequency of a mechanical
vibration filter and miniaturization of L and h at the same
scaling, that is, a decrease in L and h while being placed on the
straight line with the gradient of 1 is a sufficient condition of
the hatch portion of FIG. 34.
[0013] By miniaturizing a dimension of a mechanical vibrator in a
conventional mechanical resonator thus, a resonance frequency can
be increased. However, there was a problem that the case where a
mechanical Q value reduces and the Q value necessary to obtain
desired frequency selection characteristics cannot be obtained
occurs by miniaturizing the dimension generally.
[0014] Hence, a resonator using a single-crystal material is
contemplated as a resonator with a good Q value. In a vibrator of
the single-crystal material, internal atoms are regularly arranged,
so that a higher Q value can be obtained as compared with a
polycrystalline material. For example, FIG. 35 is a straddle
mounted beam structure fabricated by processing an SOI layer of a
silicon substrate 204 constructing an SOI (Silicon on Insulator)
substrate shown in Non-patent Reference 2. However, in this
structure, a beam can be vibrated by removing a BOX (Buried Oxide)
layer 203 under the SOI layer using hydrofluoric acid and the BOX
layer under a support part 205 is also removed and the support part
becomes brittle. Consequently, vibration of the support part 205
becomes nonnegligible and with a decrease in a resonance frequency
of a straddle mounted beam, dissipation of vibration energy from
the support part occurs, so that it becomes difficult to obtain a
large Q value.
[0015] Hence, an example of improving brittleness of the support
part 205 by thickening a thickness of the support part 205 more
sufficiently than a thickness of a vibrator 201 constructing a beam
is disclosed in Non-patent Reference 3. FIG. 36 is a structure of
the vicinity of a support part 205 of a straddle mounted beam shown
in Non-patent Reference 3. A thickness of the support part 205 is a
thickness of a silicon substrate 204 and a thickness of the beam is
thin sufficiently with respect to the thickness of the silicon
substrate and therefore, the support part 205 has a robust
structure. However, a structure of the support part is not
axisymmetrical with respect to a length direction of the beam, so
that the support part 205 becomes brittle as the support part of
one side (for example, the side A) with respect to the axis
retracts in the length direction of the beam. Since the side A and
the side A' of the support part 205 are formed by dry etching and
individual lithography of two times, high-accuracy alignment of the
lithography of two times is required in order to reduce retraction
of one side. It becomes very difficult to perform this alignment
step as a dimension of the vibrator becomes fine from the order of
.mu.m to the order of nm.
[0016] Also, in the method of fabrication of the vibrator of
Non-patent Reference 3, it is difficult to approach the vibrator
201 and form an electrode 202. FIG. 37 is a diagram showing cross
sections of the substrate and the vibrator of FIG. 36. Since there
is an opening part between the substrate and the vibrator as shown
in FIG. 37A, in the case of attempting to form the electrode 202
proximate to a side surface of the vibrator 201 by a thin-film
formation technique such as sputtering, there is the opening part,
so that the electrode cannot be anchored to the substrate or a
robust electrode structure cannot be formed because a thickness of
the electrode near to the opening part becomes extremely thin as
shown in FIG. 37B.
[0017] There is also a technique in which boron is diffused from a
surface of a silicon substrate and anisotropic etching is performed
from a back surface of the silicon substrate and a diffusion layer
of the boron is used as an etching stop layer of the anisotropic
etching (Non-patent Reference 4). When a diffusion region of boron
is formed in a beam shape, a beam type vibrator can also be formed
as shown in FIG. 38. An electrode can be formed on the substrate
surface since there is no opening part on the substrate surface
before the anisotropic etching is performed. However, an effect of
an etching stop varies depending on variations in diffusion of
boron, so that it was difficult to obtain a beam shape having a
predetermined dimension and it was extremely difficult to obtain a
desired value with respect to a resonance frequency. Also, a
surface of the vibrator is not flat, so that loss of vibration
energy resulting from surface roughness occurs and a Q value
reduces.
[0018] Also, a smaller vibrator must be formed in order to increase
a resonance frequency of a resonator to a VHF band or a UHF band.
With this, an opposite area of the vibrator and the electrode
becomes smaller, so that capacitance becomes small and also an
impedance becomes high. In a high-frequency signal, energy loss of
an RF signal increases when the extent of an impedance mismatch
becomes high. As this solution method, there is a method for
reducing an impedance by electrically connecting plural resonators
in parallel and approaching a match state. FIG. 39A is a resonator
constructed of a vibrator using aluminum as a material. A vibrator
201 has a straddle mounted beam structure and is supported by a
support part 205. An electrode 202a for excitation and an electrode
202b for detection are placed on both side surfaces of the vibrator
201 through gaps. In this configuration, the vibrator 201 produces
flexural vibration in a direction attracted in a direction of the
electrode 202a for excitation and its resonance frequency is 35.5
MHz. FIG. 39B is a configuration of electrically connecting plural
resonators of FIG. 39A in parallel and reducing an impedance.
[0019] FIG. 40A shows impedances at the time of setting the number
N of resonators electrically connected in parallel at 1, 10 and
100. However, the individual resonators are fabricated with
extremely high dimension accuracy, so that variations in a
resonance frequency become substantially 0. As shown in FIG. 40A,
the impedance can be reduced without changing resonance
characteristics as the number N is increased.
[0020] FIG. 40B shows impedances in the case of having variations
(0.3 MHz in a standard deviation) in resonance frequencies of
individual resonators. In the case of having the variations, as the
number N of resonators is increased to 10 and 100, the peak at the
resonance frequency is not sharp and it becomes difficult to
construct a good resonator.
[0021] The variations in resonance frequencies must be suppressed
by processing individual resonators with high accuracy in order to
electrically connect plural resonators in parallel for reduction in
the impedance thus. In a resonance frequency of a straddle mounted
beam, a thickness and a length of the beam become predominant in
flexural vibration and a length of the beam becomes predominant in
torsional vibration, so that it becomes important to manage the
thickness and the length of the beam in the case of using flexural
resonance and it becomes important to manage the length of the beam
in the case of using torsional resonance.
[0022] Non-patent Reference 1: Frank D. Bannon III, John R. Clark,
and Clark T.-C. Nguyen, "High-QHF Microelectromechanical Filters",
IEEE Journal of Solid-State Circuits, Vol. 35, No. 4, pp. 512-526,
April 2000
[0023] Non-patent Reference 2: Vincent Agache et. al,
"CHARACTERIZATION OF VERTICAL VIBRATION OF ELECTROSTATICALLY
ACTUATED RESONATORS USING ATOMIC FORCE MICROSCOPE IN NONCONTACT
MODE", Proc. of IEEE TRANSDUCERS '05, pp. 2023-2026
[0024] Non-patent Reference 3: A. Tixier-Mita and others, "SINGLE
CRYSTAL NANO-RESONATORS AT 100 MHz FABRICATED BY A SIMPLE BATCH
PROCESS", Proc. of IEEE TRANSDUCERS '05, pp. 1388-1391
[0025] Non-patent Reference 4: Chang-Jin Kim and others,
"Silicon-Processed Overhanging Microgripper", Journal of
Microelectromechanical Systems, Vol. 1, No. 1, 1992, pp. 31-36
DISCLOSURE OF THE INVENTION
Problems that the Invention is to Solve
[0026] While it is extremely difficult to control a thickness or a
length of a beam with high accuracy in the conventional resonator
thus, it is necessary to use vibrators having dimensions controlled
with extremely high accuracy particularly when plural vibrators are
connected and used. However, it was extremely difficult to suppress
dissipation of vibration energy of the vibrator and reproducibly
obtain the vibrator having a high Q value with high accuracy.
[0027] The invention has been implemented in view of the actual
circumstances described above, and an object of the invention is to
provide a resonator and a vibrator with a high Q value in which
dissipation of vibration energy in vibration of the vibrator is
small.
Means for Solving the Problems
[0028] In order to solve the problem described above, in a vibrator
of a resonator of the invention, a thickness of a support part of
the vibrator of a beam structure is made thicker than a thickness
of the vibrator and the support part is formed in axisymmetry with
respect to a length direction of a beam.
[0029] According to this configuration, brittleness of the support
part is improved and loss of vibration energy from the support part
can be reduced, so that a resonator having a high Q value can be
provided.
[0030] Also at the same time, a vibrator is formed by a
single-crystal material and a surface of the vibrator is formed in
a crystal plane. Consequently, loss of vibration energy resulting
from surface roughness of the surface of the vibrator can be
reduced, so that the resonator having the high Q value can be
provided. When the surface roughness is rough, the number of atoms
(a surface layer part of the vibrator) irregularly arranged
increases as compared with atoms (a deep layer part of the
vibrator) regularly arranged accordingly. Since the atoms of this
surface layer part differ from the atoms of the deep layer part in
a restricted state of motion, motion of the atoms of the deep layer
part vibrating in a state of being harmonized and arranged
regularly is inhibited, with the result that it becomes a cause of
loss of vibration energy.
[0031] That is, a vibrator of the invention is constructed of a
cavity formed in a back surface of a single-crystal substrate and a
vibrator formed so that at least one end of the vibrator is
supported by a support part within a thickness between a front
surface of the single-crystal substrate and a bottom surface of the
cavity, and a thickness of the vibrator is thinner than that of the
support part and the support part is formed in axisymmetry with
respect to a length direction of a beam.
[0032] Also, a resonator of the invention comprises a
single-crystal substrate, a cavity formed in a back surface of the
single-crystal substrate and a beam type vibrator formed so that at
least one end of the vibrator is supported by a support part within
a thickness between a front surface of the single-crystal substrate
and a bottom surface of the cavity, and an electrode which applies
excitation force of electrostatic force to the beam type vibrator,
and a thickness of the vibrator is thinner than that of the support
part and the support part is formed so as to be axisymmetrical with
respect to a length direction of a beam.
[0033] According to the configuration described above, a support
part with a film thicker than a thickness of a beam type vibrator
can be formed and also the support part can be formed in
axisymmetry with respect to a length direction of a beam, so that
the support part can be formed in a robust structure. Also,
vibration characteristics can be improved while improving support
properties. Also, by performing anisotropic etching of a
single-crystal substrate, a pattern can be formed with good
controllability and at the same time, the vibrator of a
single-crystal material can be obtained, so that loss of vibration
energy at a crystal grain boundary as seen in a polycrystalline
material can also be reduced. Also, all the surfaces of the
vibrator are formed in a crystal plane, so that there is also an
effect of reducing loss of vibration energy resulting from surface
roughness.
[0034] Also, in the resonator of the invention, a plurality of the
beam type vibrators are formed within the thickness between the
front surface of the single-crystal substrate and the bottom
surface of the same cavity.
[0035] According to this configuration, the plurality of vibrators
with the same length can be formed, so that the plurality of
resonators having the same resonance frequency can be arranged
adjacently. Therefore, by electrically connecting capacitance
formed between the electrode and the vibrators in parallel, the
capacitance is increased and an electrical impedance is decreased
and a match of the electrical impedance with an electrical circuit
of the periphery including the resonator can be obtained
easily.
[0036] Also, in the resonator of the invention, the single-crystal
substrate is a silicon substrate.
[0037] According to this configuration, a resonator can be
manufactured by a semiconductor manufacturing apparatus. Also, the
resonator can be integrated on the same silicon substrate by
connecting other active element. Or, the resonator can be
incorporated into other active element.
[0038] Also, in the resonator of the invention, the single-crystal
substrate is an SOI layer of an SOI substrate. According to this
configuration, a vibrator can be formed of a thin SOI layer with
the order of .mu.m or the order of nm, so that a beam type vibrator
having a resonance point at a very small UHF band can be
formed.
[0039] Also, in the resonator, a cross section shape of the beam
type vibrator is a triangular shape or a trapezoidal shape
surrounded by crystal planes of {100} and {111}.
[0040] In this configuration, a beam type vibrator can formed
simply using an etching solution having crystal anisotropy.
[0041] Also, in the resonator, the electrode is opposed to the
whole area of a width direction of a side surface of the beam type
vibrator exposed to a front surface of the single-crystal substrate
through an air gap, and a capacity is formed between the beam type
vibrator and the electrode.
[0042] According to this configuration, a flexural vibration mode
of a beam type vibrator can be excited by electrostatic force
generated between an electrode and the beam type vibrator.
[0043] Also, in the resonator of the invention, conductivity of the
vibrator is asymmetrical with respect to a torsional center axis of
the longitudinal direction of the beam of the vibrator.
[0044] According to this configuration, the plurality of torsional
vibrators with the same resonance frequency can be integrated
densely and an impedance can be reduced.
[0045] Also, in the resonator of the invention, the electrode is
opposed to substantially a half area of a width direction of a side
surface of the beam type vibrator exposed to a front surface of the
single-crystal substrate through an air gap, and a capacity is
formed between the beam type vibrator and the electrode.
[0046] According to this configuration, a torsional vibration mode
of a beam type vibrator can be excited by electrostatic force
generated between an electrode and the beam type vibrator. This air
gap is set at a value in which a distance between the electrode and
the vibrator can generate predetermined electrostatic force, and
the opposed electrode may be absent in the other region, and a size
of the air gap may be adjusted so that a distance to the vibrator
becomes a predetermined value or more and electrostatic force
becomes sufficiently smaller than that of the other region.
[0047] Also, in the resonator of the invention, the plurality of
electrodes in response to a resonance mode order of the beam type
vibrator.
[0048] According to this configuration, a resonator using a
high-order mode resonance frequency as well as a basic mode
frequency of vibration can be constructed.
[0049] Also, in the resonator of the invention, the electrode is
fixed on a thick film part of the single-crystal substrate arranged
at a peripheral edge of the cavity through an insulating film.
[0050] According to this configuration, a thickness of a substrate
site at which the electrode is fixed is sufficiently thicker than a
thickness of a beam, so that it is robust and thus the amount of
displacement of the electrode itself due to electrostatic force
between the electrode and the vibrator or a shock from the outside
can be reduced.
[0051] Also, in the resonator of the invention, the beam type
vibrator is a straddle mounted beam and both ends of its beam are
equipped with support parts made of impurity diffusion regions of a
conduction type opposite to that of the beam type vibrator, and an
amplifier in which the beam type vibrator is used as a channel and
the support parts made of the impurity diffusion regions are used
as a source region and a drain region is constructed.
[0052] According to this configuration, a resonance phenomenon of a
vibrator can be electrically outputted through an amplifier and
also a resonator is included inside the amplifier, so that noise
superposed on a signal rail in the case of disposing the signal
rail between the resonator and the amplifier can be reduced.
[0053] Also, the resonator of the invention includes a plurality of
resonators electrically arranged in parallel.
[0054] According to this configuration, an electrical impedance of
a resonator can further be reduced and also high dimension accuracy
can be obtained, so that characteristic variations are suppressed
and a resonator with extremely high reliability can be
obtained.
[0055] Also, in the resonator of the invention, the vibrator is
accommodated inside a case in which the atmosphere is sealed in a
vacuum.
[0056] According to this configuration, a resonator with a high Q
value in which vibration of a vibrator is not inhibited by
viscosity of air while improving protection of the resonator can be
provided.
[0057] Also, a filter of the invention includes a filter equipped
with the resonator described above.
ADVANTAGE OF THE INVENTION
[0058] According to a configuration of a resonator of the
invention, a filter with a high Q value capable of being used at a
UHF band can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0059] FIG. 1 is a perspective view of a torsional resonator in a
first embodiment of the invention.
[0060] FIG. 2 is a cross sectional view of the torsional resonator
in the first embodiment of the invention.
[0061] FIG. 3 is a cross sectional view of the torsional resonator
in the first embodiment of the invention.
[0062] FIGS. 4A and 4B are explanatory diagrams showing a
manufacturing method of the torsional resonator in the first
embodiment of the invention.
[0063] FIGS. 5A and 5B are explanatory diagrams showing a
manufacturing method of the torsional resonator in the first
embodiment of the invention.
[0064] FIGS. 6A and 6B are explanatory diagrams showing a
manufacturing method of the torsional resonator in the first
embodiment of the invention.
[0065] FIGS. 7A and 7B are explanatory diagrams showing a
manufacturing method of the torsional resonator in the first
embodiment of the invention.
[0066] FIGS. 8A and 8B are explanatory diagrams showing a
manufacturing method of the torsional resonator in the first
embodiment of the invention.
[0067] FIGS. 9A and 9B are explanatory diagrams showing a
manufacturing method of the torsional resonator in the first
embodiment of the invention.
[0068] FIGS. 10A and 10B are explanatory diagrams showing a
manufacturing method of the torsional resonator in the first
embodiment of the invention.
[0069] FIGS. 11A and 11B are explanatory diagrams showing a
manufacturing method of the torsional resonator in the first
embodiment of the invention.
[0070] FIGS. 12A and 12B are explanatory diagrams showing a
manufacturing method of the torsional resonator in the first
embodiment of the invention.
[0071] FIGS. 13A and 13B are explanatory diagrams showing a
manufacturing method of the torsional resonator in the first
embodiment of the invention.
[0072] FIGS. 14A and 14B are explanatory diagrams showing a
manufacturing method of a torsional resonator by a beam with a
trapezoidal cross section in a second embodiment of the
invention.
[0073] FIGS. 15A and 15B are explanatory diagrams showing a
manufacturing method of the torsional resonator by the beam with
the trapezoidal cross section in the second embodiment of the
invention.
[0074] FIGS. 16A and 16B are explanatory diagrams showing a
manufacturing method of the torsional resonator by the beam with
the trapezoidal cross section in the second embodiment of the
invention.
[0075] FIG. 17 is an explanatory diagram showing a manufacturing
method of the torsional resonator by the beam with the trapezoidal
cross section in the second embodiment of the invention.
[0076] FIG. 18 is an explanatory diagram showing a manufacturing
method of the torsional resonator by the beam with the trapezoidal
cross section in the second embodiment of the invention.
[0077] FIG. 19 is an explanatory diagram of a state of sealing a
resonator in a third embodiment of the invention in a vacuum.
[0078] FIG. 20 is a cross sectional view of a flexural resonator in
a fourth embodiment of the invention.
[0079] FIG. 21 is a perspective view of a resonator using
second-order flexural vibration in a fifth embodiment of the
invention.
[0080] FIG. 22 is a perspective view of a MOS transistor including
a torsional resonator in a sixth embodiment of the invention.
[0081] FIG. 23 is a cross sectional view of a configuration of FIG.
22.
[0082] FIG. 24 is a perspective view of a torsional resonator in a
seventh embodiment of the invention.
[0083] FIGS. 25A and 25B are explanatory diagrams of a
manufacturing method of the torsional resonator in the seventh
embodiment of the invention.
[0084] FIGS. 26A and 26B are explanatory diagrams of a
manufacturing method of the torsional resonator in the seventh
embodiment of the invention.
[0085] FIGS. 27A and 27B are explanatory diagrams of a
manufacturing method of the torsional resonator in the seventh
embodiment of the invention.
[0086] FIGS. 28A and 28B are explanatory diagrams of a
manufacturing method of the torsional resonator in the seventh
embodiment of the invention.
[0087] FIG. 29 is an explanatory diagram of a manufacturing method
of the torsional resonator in the seventh embodiment of the
invention.
[0088] FIG. 30 is a cross sectional view showing a modified example
of the resonator in the seventh embodiment of the invention.
[0089] FIG. 31 is a cross sectional view showing a modified example
of the resonator in the seventh embodiment of the invention.
[0090] FIGS. 32A to 32E are cross sectional views showing another
modified example of the resonator in the seventh embodiment of the
invention.
[0091] FIG. 33 is a schematic diagram showing a filter using a
conventional mechanical resonator.
[0092] FIG. 34 is a characteristic diagram showing a relation
between a higher frequency and a dimension of the mechanical
resonator in a conventional example.
[0093] FIG. 35 is an explanatory diagram of a conventional
mechanical resonator using an SOI substrate.
[0094] FIG. 36 is an explanatory diagram of a support part of a
conventional mechanical resonator using a silicon substrate.
[0095] FIGS. 37A and 37B are diagrams showing a state of forming an
electrode in the conventional mechanical resonator using the
silicon substrate.
[0096] FIG. 38 is an explanatory diagram of a method of fabrication
of a conventional vibrator in which an etch stop of a boron
diffusion region is made.
[0097] FIG. 39A is a diagram showing an example of a resonator
single body fabricated by aluminum, and FIG. 39B is a diagram
showing a configuration of electrically connecting plural
resonators in parallel.
[0098] FIG. 40A is a diagram showing a relation between an
impedance and the number of parallel connections in the case where
there are no variations in a resonance frequency, and FIG. 40B is a
diagram showing a relation between an impedance and the number of
parallel connections in the case where there are variations in a
resonance frequency.
DESCRIPTION OF REFERENCE NUMERALS AND SIGNS
[0099] 1 Vibrator [0100] 2 Electrode [0101] 3 Insulating film
[0102] 4 Substrate [0103] 5 Support part [0104] 6 Gap [0105] 7
Cavity [0106] 101, 102 Straddle mounted beam type vibrator [0107]
103 Coupling beam [0108] 104 Input rail [0109] 105 Output rail
BEST MODE FOR CARRYING OUT THE INVENTION
[0110] Embodiments of the invention will hereinafter be described
in detail with reference to the drawings.
First Embodiment
[0111] In the present embodiment, in the case of forming a vibrator
1 while forming a cavity 7 by anisotropic etching of a
single-crystal silicon substrate 4, the vibrator 1 is formed so as
to support both ends by support parts 5 within a thickness between
a front surface of the single-crystal silicon substrate 4 and a
bottom surface of the cavity 7 and a thickness of the vibrator 1 is
constructed so as to become thinner than that of the support
part.
[0112] FIG. 1 is a main perspective view of a resonator in a first
embodiment of the invention. FIG. 2 is a cross sectional view in a
vertical direction of A-A' of FIG. 1, and FIG. 3 is a cross
sectional view in a vertical direction of B-B' of FIG. 1. The
resonator of the first embodiment has a beam type vibrator 1 for
producing torsional vibration and an electrode 2 placed in
proximity to a side surface of the beam type vibrator 1 through a
gap 6.
[0113] Here, the cavity 7 is formed in a back surface of the
single-crystal silicon substrate 4 and the vibrator 1 is processed
over the cavity by the same material as that of the substrate 4.
Since the vibrator is formed by processing of the single-crystal
silicon substrate, the support part and the vibrator are
constructed of the same single-crystal material and there is no
loss of vibration energy at a crystal grain boundary surface as
seen in a vibrator of a polycrystalline material, so that the
vibrator having a high Q value can be formed.
[0114] Both ends of the vibrator 1 are fixed to the support parts
5. A thickness of the support part 5 is equal to a thickness of the
substrate 4 and is thicker than a thickness of the vibrator 1.
Therefore, as compared with the case where the thickness of the
support part is equal to the thickness of the vibrator, the support
part becomes robust and even when the vibrator 1 vibrates, the
support part is resistant to being vibrated and an effect of
suppressing the amount in which vibration of the vibrator 1 leaks
to the support part and vibration energy dissipates is obtained.
Also, since the vicinities of connection between the vibrator 1 and
the support parts 5 are symmetrical with respect to the axis of
B-B' of a length direction of the vibrator 1, one side of the
support part 205 does not retract in the length direction of the
beam as shown in FIG. 36 of the conventional example, so that the
support part has a more robust structure.
[0115] The electrode 2 is formed by a polycrystalline silicon film.
As shown in FIG. 2, the electrode 2 is opposed to about a half area
of a width direction of a side surface of the beam type vibrator 1
through a gap 6, and a capacity is constructed between the beam
type vibrator 1 and the electrode 2. This is because electrostatic
force effectively provides the torsional rotation moment when the
electrostatic force is applied to a gap between the opposed
surfaces and the vibrator produces vibration.
[0116] Next, a manufacturing method of the resonator in the first
embodiment of the invention will be described using FIGS. 4 to 13.
In FIGS. 4 to 13, A is a diagram in the case of viewing a main part
from the above of a substrate and B is a cross sectional diagram of
C-C' of A (C-C' is omitted in FIGS. 5 to 13). The substrate 4 is a
single-crystal silicon substrate, and a front surface and a back
surface of the substrate are a plane of {100} and a surface viewed
in a cross section is a plane of {110}. A silicon oxide film is
formed on the front surface (lower side in the drawings) of the
substrate. A silicon nitride film is formed on the back surface of
the substrate and a quadrilateral window is formed in this silicon
nitride film. Four sides of this window are formed along a plane of
{111}.
[0117] Anisotropic etching of silicon is performed from the back
surface of the substrate 4 using KOH. An etching rate of the plane
of {111} is lower than those of the other planes, with the result
that the etching proceeds so as to expose the plane of {111} as
shown in FIGS. 5A and 5B. {100} and {111} have a position relation
of 54.7.degree.. The etching is stopped within the time when a
bottom surface of the cavity 7 does not reach the front surface of
the substrate. When the silicon nitride film is removed, a state of
FIGS. 6A and 6B is obtained.
[0118] Next, it proceeds to a fabrication step of a beam type
vibrator. A processing technique by anisotropic etching is also
used in fabrication of a beam structure. This is introduced in
Non-patent Reference 5, and partially utilizes a method also used
in generation of a quantum wire. In FIGS. 7A and 7B, a silicon
nitride film 8 is again deposited on the back surface of the
substrate 4 and a quadrilateral window is formed in the silicon
nitride film 8. At this time, one side of the window traverses the
cavity 7 and is formed along a plane of {111} as shown in FIGS. 7A
and 7B.
[0119] Anisotropic etching of silicon is performed from the back
surface of the substrate 4 using KOH. When the etching proceeds
until the front surface of the silicon substrate 4 is reached, the
state is shown in FIGS. 8A and 8B.
[0120] Next, a local silicon oxide film (LOCOS) is formed on the
portion in which silicon is exposed to the back surface of the
silicon substrate 4 (FIGS. 9A and 9B). For example, a silicon
surface is oxidized by an oxidation furnace. FIGS. 10A and 10B show
a state in which the LOCOS is left and the silicon nitride film is
removed.
[0121] When anisotropic etching of silicon is again performed using
KOH using the LOCOS as a mask, a vibrator of a straddle mounted
beam with a triangular cross section can be formed as shown in
FIGS. 11A and 11B. Three side surfaces of the beam are formed by
two planes of {111} and one plane of {100}.
[0122] Then, as shown in FIG. 12, with respect to the front surface
of the silicon substrate 4, an electrode 2 is deposited on a
silicon oxide film 3 and is patterned. A polycrystalline silicon
film formed by, for example, a CVD (chemical vapor deposition
method) is used in the electrode 2. This patterning is formed so
that the electrode 2 is opposed to a beam side surface to about a
half the width of the beam. This is effective in the case of using
the beam type vibrator 1 as a torsional vibrator. This is because
the torsional moment is effectively caused to act on the vibrator 1
by electrostatic force between the vibrator 1 and the electrode
2.
[0123] Finally, the silicon oxide film 3 is removed. This can be
removed using, for example, hydrofluoric acid. Consequently, as
shown in FIG. 13, a gap 6 is formed between the vibrator 1 and the
electrode 2 and the vibrator 1 becomes a state capable of vibration
and a capacity is formed between the vibrator 1 and the electrode
2. FIG. 13 is equal to the configuration of FIGS. 1 to 3.
[0124] Non-patent Reference 5: G. Hashiguchi and H. Mimura,
"Fabrication of Silicon Quantum Wires Using Separation by Implanted
Oxygen Wafer", Jpn. J. Appl. Phys, Vol. 33 (1994), pp.
L1649-1650
[0125] According to the above manufacturing method, as shown in
FIGS. 1 to 3, a thickness of the support part 5 of the beam type
vibrator 1 can be made thicker than a thickness of the beam and the
support part can be formed robustly. Also, since the vicinities of
connection between the vibrator and the support parts are
symmetrical with respect to the axis of B-B' of a length direction
of the vibrator 1, one side of the support part does not retract in
the length direction of the beam as shown in FIG. 36 of the
conventional example, so that the support part has a more robust
structure. Therefore, the amount in which vibration of the vibrator
1 leaks to the support part can be reduced and a resonator having a
high Q value can be constructed.
[0126] Also, since a material of the beam type vibrator 1 is the
same single-crystal material as the substrate 4, there is no loss
of vibration energy at a crystal grain boundary surface as seen in
a vibrator of a polycrystalline material, so that the resonator
having a high Q value can be constructed.
[0127] Also, since a side surface of the beam type vibrator 1 is
constructed of a crystal plane of silicon, a surface texture is
extremely smooth and loss of vibration energy resulting from
surface roughness is reduced and the resonator having a high Q
value can be constructed.
[0128] Also, the manufacturing method shown in FIGS. 4 to 13 is a
manufacturing method useful for fabrication of a fine beam type
vibrator having a resonance frequency at several hundreds MHz to
several GHz since a structural object with thinness exceeding a
limit of a patterning width in a semiconductor process can be
formed. For example, when a length of a beam is set at 1.6 .mu.m, a
vibrator having a torsional resonance frequency at 1.2 GHz can be
provided.
[0129] In addition, TMAH (Tetramethyl Ammonium Hydroxide) or EDP (a
mixture of water, pyrocaterol, ethylenediamine) instead of KOH may
be used in the anisotropic etching.
Second Embodiment
[0130] In addition, a cross section of the beam type vibrator 1 is
formed in the triangular shape, but can also be formed in a
trapezoidal shape. A manufacturing method of a resonator having
this trapezoidal-shaped vibrator will be described as a second
embodiment of the invention.
[0131] In the present embodiment, a vibrator with a trapezoidal
cross section can be formed by being fabricated similarly from
FIGS. 4 to 6 and being changed subsequently as shown in FIGS. 14A
and 14B. In FIGS. 14A and 14B, a silicon nitride film is deposited
on a back surface (upper side in the drawing) of a substrate 4 and
two quadrilateral windows are formed in the silicon nitride film.
At this time, the linear silicon nitride film 8 having a certain
width traverses a cavity 7 and is formed along a plane of {111}.
When anisotropic etching of silicon is performed using KOH, a
vibrator of a straddle mounted beam with a trapezoidal cross
section can be formed as shown in FIGS. 15A and 15B. Four side
surfaces of the trapezoid are formed by two planes of {111} and two
planes of {100}. FIGS. 16A and 16B are a state in which the silicon
nitride film is removed, and FIG. 17 is a state in which an
electrode 2 of polycrystalline silicon is formed on a silicon oxide
film 3 on a front surface (lower side in the drawing) of the
silicon substrate 4 and is patterned. This patterning is formed so
that the electrode 2 is opposed to about a half the width of the
beam. This is because the torsional moment is caused to act on the
vibrator 1 by electrostatic force between the vibrator 1 and the
electrode 2, and a torsional resonator can be constructed by this
configuration. Finally, the silicon oxide film is removed (FIG.
18). This can be removed using, for example, hydrofluoric acid. A
gap 6 is formed between the vibrator 1 and the electrode 2 and the
vibrator 1 becomes a state capable of vibration. The steps of FIGS.
14 to 18 are characterized in that the resonator can be formed by
the number of steps smaller than the steps of FIGS. 4 to 13 in
which the triangular cross section is obtained.
Third Embodiment
[0132] Also, this resonator may be constructed so as to be
accommodated inside a case in which the atmosphere is sealed in a
vacuum as shown by one example in FIG. 19. In this case, a silicon
substrate 4 is bonded to a silicon base 11 through an adhesive
layer 10 such as an adhesive. Further, the resonator is included by
a recessed part of a glass cap 9 and the silicon base 11, and the
glass cap 9 is bonded to the silicon base 11 by anode bonding while
vacuuming the inside. By this configuration, the resonator for
reducing loss of vibration energy of a vibrator 1 by viscosity of
air can be provided while the resonator with high protection can be
formed.
Fourth Embodiment
[0133] Also, a flexural resonator can be constructed by changing a
configuration of the electrode of the torsional resonator shown in
FIGS. 1 to 3. FIG. 20 shows a cross sectional view of the flexural
resonator. An electrode 2 is opposed to the whole area of a width
direction of a vibrator 1 through a gap and a capacity is formed.
By the present configuration, excitation force in which the
vibrator 1 flexurally vibrates in a substrate thickness direction
can be applied by electrostatic force.
Fifth Embodiment
[0134] Also, a high-order resonance mode can be excited with
respect to a vibrator since there are plural electrodes according
to a resonance mode order which wants to be excited with respect to
the vibrator. FIG. 21 is a configuration in which the number of
electrodes of the torsional resonator of FIG. 1 is two. Electrodes
2a, 2b are positioned at a site at which excitation force is
applied to two antinodes of a second-order torsional mode and its
vicinity in order to excite second-order torsional vibration of a
beam. That is, the portion in which the electrodes 2a, 2b are
opposed to a side surface of the beam is set at the site of about a
half the length of the beam and about a half the width of the
beam.
[0135] Also in this case, a uniform beam having extremely high
dimension accuracy can be formed, so that a resonator with high
reliability can be provided.
Sixth Embodiment
[0136] Also, a configuration of fetching an output signal from a
resonator and amplifying the signal can be added in substantially
the same configuration as FIG. 1. FIG. 22 newly disposes two
electrodes on support parts 5 of both ends of a vibrator 1 in the
resonator of FIG. 1. An electrode on the vibrator 1 is set at a
gate electrode 22 and one electrode on the support part 5 is set at
a drain electrode 24 and the other electrode is set at a source
electrode 23. FIG. 23 is a longitudinal cross sectional view of
B-B' of FIG. 22. A substrate 4 and the vibrator 1 are an N-type
semiconductor, and a drain region 26 made of a p+ diffusion region
is formed in the substrate under the drain electrode 24 and a
source region 25 made of a p+ diffusion region is formed in the
substrate under the source electrode 23. That is, the whole
resonator is a p channel MOS transistor and an oxide film of a MOS
structure is replaced with a gap 6 and vibration of the vibrator 1
is enabled. The vibrator 1 is subjected to electrostatic force
between the vibrator 1 and the gate electrode 22 and torsional
vibration of large amplitude is produced in the vicinity of a
resonance frequency and modulation is given to formation of a
channel of the inside of the vibrator 1, with the result that a
drain current involved is obtained. According to such a
configuration, as compared with the case of individually
fabricating a resonator and an amplifier and making connection
between both of the resonator and the amplifier by wiring, an
element can be miniaturized and also superposition of noise or loss
resulting from the wiring can be reduced.
[0137] A gate vibration type MOSFET for vibrating a gate electrode
has been devised conventionally, but polycrystalline silicon or a
metal thin film is normally used in a gate electrode material, so
that loss of vibration energy at a crystal grain boundary occurs
and a resonator with a high Q value cannot be constructed, but by
constructing a vibrator of single-crystal silicon thus, the
vibrator corresponding to a channel constructed of the
single-crystal silicon vibrates, so that an amplifier and a
resonator with a high Q value can be formed.
[0138] Also, in the present embodiment, as a material of the
substrate 4, silicon is used, but a semiconductor material such as
SiGe can also be used.
[0139] Also, by electrically placing the resonators of the present
embodiment in parallel, an electrical impedance of the resonator is
reduced and electrical impedance matching between the resonator and
a signal circuit of the outside of the resonator can be
improved.
[0140] Also, the vibrator of the present embodiment is formed by
processing the silicon substrate, but the vibrator may be formed by
processing an SOI layer of an SOI (Silicon On Insulator) substrate.
Since there is also the SOI substrate having a thin SOI layer with
the order of .mu.m or the order of nm, it can be used in the case
of forming a beam type vibrator having a resonance point at a very
small UHF band with an extremely thin thickness and a short
length.
Seventh Embodiment
[0141] Next, a seventh embodiment of the invention will be
described. In the torsional resonator of FIG. 1 in the first
embodiment, one vibrator is formed in the bottom of the cavity, but
plural vibrators can be formed simultaneously. FIG. 24 shows a
perspective view of the case of forming two vibrators. With respect
to each of the vibrators, electrodes 2a, 2b are placed through
gaps. In this case, it is a torsional resonator, so that both the
electrodes 2a, 2b are formed so as to oppose the electrodes to the
vibrators to about a half area of a width direction of the beam
type vibrator and are configured to effectively excite torsional
vibration.
[0142] The resonator can be formed by being fabricated similarly in
the steps of FIGS. 4 to 6 and being changed subsequently as shown
in FIG. 25. In FIGS. 25A and 25B, a silicon nitride film is
deposited on a back surface (upper side in the drawing) of a
substrate 4 and two quadrilateral windows are formed in the silicon
nitride film. At this time, the linear silicon nitride film having
a certain width traverses a cavity and is formed along a plane of
{111}. When anisotropic etching of silicon is performed using KOH,
the state is shown in FIGS. 26A and 26B. A LOCOS is formed on the
portion in which silicon is exposed to the back surface of the
substrate 4 (FIGS. 27A and 27B) and the silicon nitride film is
removed (FIGS. 28A and 28B) and when anisotropic etching of silicon
is again performed using KOH using the LOCOS as a mask, two beam
type vibrators with triangular cross sections can be arranged and
formed as shown in (FIG. 29).
[0143] Since a plane a of {111} and a plane b of {111} shown in
FIGS. 28A and 28B are crystal planes, two beam type vibrators
having these planes on end faces become the same length with high
accuracy. That is, resonance frequencies of the two beam type
vibrators become the same. A resonance frequency of flexural
vibration depends on a thickness and a length of the beam, but
particularly in a resonance frequency of torsional vibration,
thickness dependence is extremely smaller than length dependence,
so that the present configuration serves as effective means in the
case of forming plural vibrators having the same torsional
resonance frequency.
[0144] In the configuration shown in FIG. 24, as the vibrator
becomes finer, capacitance between the electrode and the vibrator
becomes small and an electrical impedance becomes large and a
mismatch of the electrical impedance tends to occur and energy of
an input AC signal becomes resistant to being converted into
mechanical vibration of the vibrator efficiently. However, the
impedance can be decreased by connecting plural pairs of the plural
electrodes and vibrators having the same resonance frequency in
parallel as shown in FIG. 24.
[0145] In addition, FIG. 30 shows a configuration of the case of
forming more vibrators having the same resonance frequency. FIG. 30
is a cross sectional view in which a cross section of a beam is
seen. A feature herein is that a first cavity C1 is formed.
Subsequently, a method of formation of the vibrators shown in FIGS.
4 to 6 and FIGS. 25 to 29 is performed with respect to a bottom
surface of the first cavity C1. Finally, the first cavity C1 and a
second cavity C2 are formed in a substrate 4 and plural vibrators
are formed between a bottom surface of the second cavity C2 and a
front surface of the substrate 4. According to the present
configuration, an electrode 2 is robustly fixed while the plural
vibrators with the same length can be formed. That is, a thickness
Z0 between the bottom surface of the first cavity and the front
surface of the substrate 4 can be made sufficiently thicker than a
thickness Z1 of the beam, so that the electrode 2 can be formed at
a robust site on the substrate. Consequently, the amount of
displacement of the electrode itself due to electrostatic force
between the electrode and the vibrator or a shock from the outside
can be reduced.
Eighth Embodiment
[0146] Next, FIG. 31 shows another configuration of a low-impedance
resonator using a torsional vibration mode as an eighth embodiment
of the invention. In FIG. 31, plural vibrators are formed with
respect to a bottom surface of a first cavity in a manner similar
to FIG. 24, and the low-impedance resonator of FIG. 31 differs from
the low-impedance resonator of the seventh embodiment shown in FIG.
24 in that an electrode 2 is formed so as to be opposed to the
whole area of a width direction of the vibrator rather than about a
half area of the width direction of the vibrator. Then, it is
characterized by being constructed so that electrical
characteristics of a part of the vibrators opposed to this
electrode 2 differ from those of the other parts and electrostatic
force in a part of the electrode 2 substantially becomes larger
than that in the other parts. In order to change the electrical
characteristics herein, it is characterized by being constructed so
that an impurity diffusion region 1d is formed in only a part of
the region opposed to the electrode 2 among the vibrators and large
electrostatic force is generated in a region in which this impurity
diffusion region 1d is opposed to the electrode 2. Therefore,
patterning of the electrode 2 as shown in FIG. 24 is not required.
Also, the portion of the thickness Z0 for supporting the electrode
2 is not required in FIG. 30, so that the vibrators tend to be
integrated at a higher density.
[0147] Conductivity of the vibrator is set in asymmetry with
respect to the torsional center axis of a longitudinal direction of
a beam in order to excite a torsional vibration mode in FIG. 31.
FIG. 32 shows an example of a manufacturing step of giving
asymmetrical properties to the conductivity. FIG. 32A is
substantially similar to FIG. 14 which is the explanatory diagram
of fabricating the beam with the trapezoidal cross section
described in the second embodiment, but the number of patterns of a
silicon nitride film mask formed on a bottom surface of a cavity is
set at plural patterns. When anisotropic etching is performed on
the bottom surface of the cavity, the state is shown in FIG. 32B.
Here, while leaving the silicon nitride film mask, impurity ions of
one conduction type such as phosphorus are implanted into a silicon
surface exposed and annealing is performed and thereby, an impurity
diffusion region is formed (FIG. 32C). In the case of forming the
impurity diffusion region, SOG (spin on glass) or ion implantation
may be used as a diffusion source. Next, when anisotropic etching
is again performed after a protective film of a silicon oxide film
is formed on the silicon surface into which the impurities are
diffused and the silicon nitride film is removed, a beam with a
triangular cross section having asymmetrical properties in
conductivity is formed as shown in FIG. 32(d). Here, FIG. 32(d) is
an enlarged view. When an electrode 2 is formed on a silicon oxide
film 3 (lower side of the silicon oxide film in the drawing) and
the silicon oxide film 3 is removed by hydrofluoric acid, a
structure of FIG. 32(e), that is, the low-impedance resonator using
the torsional vibration mode shown in FIG. 31 is obtained. When a
voltage is applied between the vibrators and the electrode, as
shown in FIG. 32(e) by asymmetrical properties of conductivities of
the vibrators, the voltage is applied between the electrode and a
site with high conductivity, that is, the impurity diffusion region
and electrostatic force is generated and thereby, rotary excitation
force (moment) is selectively applied to this impurity diffusion
region and the torsional vibration mode can be excited.
[0148] By the above, in the configuration of FIG. 31, the lengths
of the beams are the same, so that the plural vibrators without a
resonance frequency drift can densely be placed in parallel inside
the same cavity and a low impedance can be achieved and a high-Q
resonator using a torsional vibration mode with a high Q value can
be implemented.
[0149] In addition, the embodiment described above is constructed
so as to partially change conductivity of a vibrator by diffusing
impurities of one conduction type such as phosphorus, but torsional
vibration can also be generated by insulating a part of the
vibrator by implantation of oxygen ions etc. In this case, an
implanted region of the oxygen ions is insulated and becomes a
region in which electrostatic force is resistant to being generated
and thereby, rotary excitation force (moment) is applied to a
region in which the oxygen ions are not implanted, and the
torsional vibration mode can be excited.
INDUSTRIAL APPLICABILITY
[0150] A resonator according to the invention is constructed so
that an extremely fine structural body capable of being fabricated
in a semiconductor process is excited by electrostatic force
mainly, and particularly with respect to a beam type vibrator, a
support part is formed in a robust structure and a surface of the
vibrator is formed in a flat crystal plane and thereby, a resonator
with a high Q value in which dissipation of vibration energy is
reduced is provided. The present resonator is useful as a
high-frequency circuit etc. integrated at a high density and loaded
into a portable wireless terminal. Also, it can be applied to uses
of medical or environmental fields etc. of a mass analysis etc. by
mechanical resonance or a spectral analysis in an ultrasonic band
or a voice band.
* * * * *